1. Field of the Invention
This invention relates to an improved wire saw of the type advantageously used in the sawing of various materials, particularly hard materials, e.g., glass, ceramics, refractories, concrete and stone such as blocks of granite, marble, limestone and the like. More specifically, it relates to an improved wire saw employing fixed or bonded abrasives which is characterized by high strength, improved efficiency and enhanced wear characteristics.
While the present invention will be described with particular reference to certain embodiments employing fixed super-abrasive particles, which may be advantageously employed in the sawing of granite, it should be understood that the invention is not necessarily limited thereto. The concept set forth herein can be readily adapted for use in connection with other embodiments and other applications, as those skilled in the art will recognize in the light of the present disclosure.
As used herein, the term "super-abrasive" refers to abrasive media having a hardness on the Knoop scale in excess of about 3000 kg/mm.sup.2. Commercially-available super-abrasives include natural and synthetic industrial diamonds and cubic boron nitride, although the present invention is not necessarily limited thereto. These contrast with conventional abrasives which have a hardness substantially less than about 3000 kg/mm.sup.2, e.g., garnet, tungsten carbide, silicon carbide, emery, aluminum oxide and the like. A comparison of Knoop and Mohs hardness values for conversion purposes is available in standard handbooks.
2. Description of the Prior Art
As those skilled in the art of stonecutting are well aware, there are many types of wire saws available for sawing blocks of granite, marble, limestone and the like into slabs or other desired geometric configurations. These prior-art saws include, for example, stranded wires, twisted metal strips, and the like which are used in conjunction with abrasive slurries to achieve the desired cutting action. Illustrative patents featuring such saws include, for example, U.S. Pat. Nos. 1,730,756, 2,003,994, 2,123,619, 2,451,383, 2,604,883, 2,718,222, 2,876,761 and 3,532,083.
While certain of such saws are used extensively, they suffer from one or more limitations. They may, for example, have limited tensile strength which prevents the use of the higher tensile forces required to produce faster, more efficient cuts. Because super-abrasives are so costly and the loss rates of abrasives when used in the form of slurries is so high, such slurry techniques are largely limited to conventional abrasives with their inherently limited sawing characteristics. Moreover, the usable life of such saws is extremely limited, particularly when sawing the harder stones such as granite. To achieve extended sawing cycles, they must be used in excessively long lengths, sometimes substantially exceeding a thousand feet or more in total loop length. Because the abrasives are used in slurry form, handling and ecological problems result which limit and/or increase the cost of using the same.
Alternative designs include stranded wires which provide the carrier or substrate for bonded abrasives, illustrative prior art including, for example, U.S. Pat. Nos. 640,139, 1,306,636 and 3,150,470. While these wire saws avoid the problems associated with abrasive slurries, the design thereof also precludes maximum tensile sawing forces because of the substantial proportion of interstitial void spaces in the wire cross section. Increasing the overall diameter of the stranded wire to compensate for the presence of undesired void spaces is counter-productive because the width of the resulting kerf is correspondingly increased. Thus, the higher tensile sawing forces are largely offset by the increased amount of stone which has to be sawn. This also represents a loss of materials and an increase in the amount of swarf which has to be removed and disposed of.
Attempts have also been made to use a single solid wire carried or substrate for the abrasive and thereby maximize the cross-sectional area carrying the tensile forces within the constraint of a given diameter. Illustrative prior art includes, for example, U.S. Pat. Nos. 2,633,681 and 3,886,926. While representing an advantageous alternative in some respects, such single wire designs suffer from the fact that the large diameter or maximum diametral dimension of the single wire results in high bending stresses therein as the carrier passes around the various drive and guide sheaves during the sawing operation. Since the total stress to which a wire may be subjected before rupture or fatigue failure is a substantially additive function of bending stresses, tensile sawing stresses and other miscellaneous stresses, the large bending stresses associated with the large diameter of a single strand severely limit the useful tensile sawing stresses or forces which may be applied. In short, the single strand alternative suffers from a relatively high and unfavorable ratio of unavoidable bending stresses to maximum permissible tensile sawing stresses. The end result is limited sawing rates and shortened saw life and attendant inefficiency and high cost.
Moreover, in certain single solid wire designs, the advantage of maximized cross-sectional areas may be further offset in part by a departure from a generally circular cross section and the presence of load-robbing grooves or slots therein. Since the bending stresses are essentially determined by the largest diametral dimension, the loss in cross-sectional area due to the presence of slots or the like is not accompanied by a corresponding decrease in bending stresses, which continue to unduly limit the tensile sawing forces which can be applied. The inherent lack of flexibility in a single strand also makes it difficult to handle, store or process, particularly in extended lengths.